Unfortunately the 258 won’t fit. These early Americans are really 1950 Nash Ramblers under the skin and space under the hood is extremely limited. (The old 195.6 engine was much shorter than the newer AMC/Rambler/Jeep six.)

A modern 4-banger should fit in there but it obviously would require a lot of custom work. You might even be able to fit a V6 but it would probably entail some surgery since the inner fender structure intrudes deeply into the engine compartment.

There’s basically nothing other than the original type engine that will easily drop into these cars.

Yes, in fact the OHV 195.6 was a factory option in the American starting I think in 1961. It was an adaptation of the old flathead engine and fits right in its place. The “hot” setup for these cars was the OHV engine with a 2-barrel carb. These engines had only 4 main bearings which is probably one of the reasons why they’re so much shorter than the newer, 7-main-bearing AMC sixes.

Here’s a video of a ’63 American “440H” with the OHV engine. You can see inside the engine compartment at about the two minute mark and it becomes immediately obvious that there’s not much room in there for an alternative engine:

I believe that the flathead 195.6 has the distinction of being the last such engine to be used in an American car. (It was the Rambler American base engine through the 1965 model year. Yes, in 1965 you could buy a car from American Motors that came equipped with a flathead engine, trunnion front suspension, and vacuum windshield wipers!)

Yes, there is a reason why the greenhouse seems kind of narrow and upright with a very vertical windshield over the squarish body. It’s because the basic structure is the roundish early fifties Rambler. After being out of production for a few years it was brought back (1960?) with the tail lights turned upside down and the wheel openings enlarged. It was obviously an old design at that point. Then the do over squarish model, but based on the old structure.

Yep, that’s pretty much the story. The original 100-inch wheelbase Nash Rambler first debuted in 1950 and was discontinued in 1955. Then it was brought back with minor tweaking as the Rambler American in 1958. The underlying 1950 Nash was used as the basis of the American through the 1963 model year.

It’s just paint. The trim seems unscathed (so far) and it’s really nothing that can’t be reversed, so I don’t have a problem with it. I don’t love the look, but that’s 100% the owner’s prerogative and 0% mine.